This invention relates to the production and recovery of hydrogen from partial oxidation of carbon-bearing feedstocks and the reaction of carbon monoxide from the partial oxidation reactor with steam in a slurry reactor containing an iron-based catalyst to produce higher hydrocarbons and carbon dioxide.
The emergence of fuel cells as a non-polluting means for producing electrical power on a large scale and the need to reduce the sulfur and aromatic content of gasoline and Diesel fuel have increased the demand for hydrogen. The conventional method for producing hydrogen is steam reforming of natural gas. Carbon monoxide formed along with the hydrogen can be reacted with steam to produce additional hydrogen and carbon dioxide (water gas shift reaction). Other feedstocks such as coal, petroleum coke, and heavy oil from oil refinery operations when reacted with water or steam and oxygen produce a mixture of hydrogen, carbon monoxide and carbon dioxide. The water gas shift reaction can be used to produce more hydrogen from the carbon monoxide. Therefore, in order to maximize hydrogen production in the aforementioned processes, wherein the water gas shift reaction is employed, the carbon in the feedstock is contained in carbon dioxide which can be sequestered, sold as a product or vented to the atmosphere. Concern over the potential contribution of atmospheric carbon dioxide to global warming has created the need to minimize production and venting of carbon dioxide. This invention reduces the amount of carbon dioxide produced by replacing the water gas shift reactor with a Kolbel-Engelhardt (K-E) reactor to produce valuable hydrocarbon products.
The Kolbel-Engelhardt reaction uses steam instead of hydrogen to react with carbon monoxide over a dual function catalyst to produce primarily hydrocarbons having at least five carbon atoms and carbon dioxide. The K-E reaction is best carried out in a three-phase bubble column reactor containing an iron-based powdered catalyst suspended in wax. This technique is an improvement over the aforementioned conventional technology wherein the carbon monoxide is reacted with water in a shift reactor to produce hydrogen and carbon dioxide. (See xe2x80x9cA Single IGCC Design for Variable CO2 Capturexe2x80x9d, by Luke F. O""Keefe et al, Fifth European Gasification Conference, April 2002). It can be appreciated by those skilled in the art, that in the conventional technology all of the carbon is removed in carbon dioxide, whereas in the instant case a portion of the carbon is removed in the hydrocarbons produced in the K-E reactor.
H. Dreyfus in U.S. Pat. No. 2,257,293 teaches a method of manufacturing hydrocarbons using a reactor comprising two zones. The first zone is used to carry out the water gas shift reaction to produce hydrogen and carbon dioxide and the second zone is used to carry out the Fischer-Tropsch reaction between hydrogen and carbon monoxide In U.S. Pat. No. 2,497,964 Sumerford teaches a method for adding steam to a fluidized-bed Fischer-Tropsch reactor to minimize carbon formation. In U.S. Pat. No. 2,579,663 Gilbert and Montgomery teach a method of reacting steam and carbon monoxide to produce normally liquid hydrocarbons. In U.S. Pat. No. 2,786,863 Kolbel and Engelhardt teach a method for producing oxygen-containing compounds using a mixture of steam, carbon monoxide and methanol.
The reaction between water and carbon monoxide to produce hydrocarbons using a powdered iron-based Fischer-Tropsch catalyst in a slurry reactor was reported by Kolbel and Ralek in xe2x80x9cThe Fischer-Tropsch Synthesis in the Liquid Phasexe2x80x9d, Catal. Rev.-Sci. Eng., 21(2), 258 (1980). This reaction was referred to as the Kolbel-Engelhardt (K-E) reaction.
Louis deVries, (U.S. Pat. No. 5,714,657), describes a process comprising partial oxidation (POX) of natural gas followed by a reaction between hydrogen and carbon dioxide (reverse water gas shift) to produce water and carbon monoxide. The water and carbon monoxide are reacted in a K-E reactor to produce normally liquid hydrocarbons. Hydrogen and carbon dioxide from the K-E reactor are recycled back to the reverse water gas shift reactor via the POX reactor to produce additional carbon monoxide and water. In U.S. Pat. No. 6,416,568 Wallace et al teach a method for recovering and purifying a high pressure stream of hydrogen separated from synthesis gas using a membrane. Gases not permeating through the membrane containing carbon monoxide are used as fuel in a gas turbine/generator set to produce electrical power.
Hydrogen, liquid hydrocarbons and carbon dioxide are produced from feedstocks such as natural gas, coal, petroleum coke, or oil refinery bottoms. A feedstock is reacted with oxygen and water (and/or steam) to produce a mixture of hydrogen and carbon monoxide in a partial oxidation reactor (POX). Partial oxidation technologies developed by Texaco, Shell, Lurgi and others are suitable for use with this invention. For example, POX reactors described in U.S. Pat. No. 3,544,291 by W. G. Schlinger et al and in U.S. Pat. No. 4,022,591 by Gernot Staudinger can be used. The oxygen can be essentially pure as obtained from an air separation unit or diluted with nitrogen and argon as in air. Air separation can be carried out using either cryogenic or pressure swing adsorption systems, both of which are available commercially. The hot gases leaving the POX unit-are mainly hydrogen, carbon monoxide, carbon dioxide and water and are cooled either by water quench or in a waste heat boiler. Typically if the feedstock is natural gas or other feedstock having little or no ash, a waste heat boiler can be used. Sulfur compounds and carbon dioxide are removed by conventional technologies using chilled methanol, hot potassium carbonate, or amine systems. Hydrogen, the first product, is removed from the remaining carbon dioxide and carbon monoxide using a membrane or other separation means such as pressure swing adsorption. The carbon monoxide is reacted with steam over a dual-function catalyst to produce liquid hydrocarbons and carbon dioxide (the Kolbel-Engelhardt reaction). The bi-functional catalyst must possess activity for the water gas shift reaction and for the Fischer-Tropsch (FT) reaction since the hydrogen for the FT reaction comes from the water gas shift reaction.
The diameter of the catalyst particles should be between 1 and 150xcexc for best results. Catalyst prepared in accordance with U.S. Pat. No. 5,504,118 by Benham et al is suitable for this application and is incorporated herein by reference. Alternatively, the catalyst prepared according to U.S. Pat. No. 5,504,118 can be strengthened by addition of a bonding agent.
The K-E reactor can be operated within a temperature range of 450 to 550xc2x0 F., more preferably between 460 and 520xc2x0 F. and most preferably between 480 and 500xc2x0 F. The K-E reactor can be operated at pressures in the range of 150-500 psia, more preferably between 200 and 400 psia, and most preferably between 300 and 375 psia. The K-E reactor can be operated at a space velocity in the range of 2-10 normal liters (NI) of combined steam and CO per hour per gram of catalyst, more preferably between 2.5 and 7.5 NI of steam and CO per hour per gram of catalyst, and most preferably between 3.0 and 5.0 NI steam and CO per hour per gram of catalyst. The superficial velocity of the gases fed to the K-E reactor should be in the range of 2 to 20 cm/s, more preferably between 5 and 15 cm/s and most preferably between 8 and 12 cm/s. The superficial velocity is defined as the velocity that the combined gases fed to the K-E reactor would have in an empty reactor at the temperature and pressure of the gases at the inlet to the reactor. The volumetric ratio of steam to carbon monoxide flow rates should be between 0.5 and 2, more preferably between 0.75 and 1.5, and most preferably between 0.9 and 1.1.
In order to maintain the slurry level in the K-E reactor constant and to keep the catalyst inventory in the reactor constant, a catalyst/wax separation device must be employed. The catalyst/wax separation device described in co-assigned U.S. Pat. No. 6,068,760 to Benham et al is suitable for use in this invention and is incorporated herein by reference.
The partial oxidation reaction, can be expressed as:
CHz+0.50 O2 z/2H2+COxe2x80x83xe2x80x83(1) 
where z is the H:C ratio of the feedstock.
The water gas shift reaction also takes place:
H2O+CO H2+CO2xe2x80x83xe2x80x83(2) 
Where x represents the number of moles of water (or steam) reacted per mole of carbon in the feedstock, then equations 1 and 2 can be combined to give:
CHz+0.5 O2+xH2O (x+z/2)H2+(1xe2x88x92x)CO+xCO2xe2x80x83xe2x80x83(3) 
The desired H2:CO ratio exiting the gasifier dictates the amount of water (or steam) reacted with the feedstock. For example, if r represents the desired H2:CO ratio, then
x=(rxe2x88x92z/2))/(1+r)xe2x80x83xe2x80x83(4) 
The number of moles of water exiting the gasifier is determined by the equilibrium constant for the water gas shift reaction 2. If K designates the equilibrium constant, then the number of moles of water exiting the gasifier per mole of carbon gasified is:
NH20=[(x+z/2)x]/[K(1xe2x88x92x)]xe2x80x83xe2x80x83(5) 
The Kolbel-Engelhardt reaction requires the water gas shift reaction (Eq. 2 above) to provide the H2 for the Fischer-Tropsch reaction:
2H2+CO CH2+H2Oxe2x80x83xe2x80x83(6) 
Combining equations 2 and 6 gives:
uH2O+CO (1xe2x88x92u)/2CH2+(1+u)/2CO2+(3uxe2x88x921)/2H2xe2x80x83xe2x80x83(7) 
where u is the H2O:CO usage ratio which has a value of ⅓ for the case where no H2 is fed to the K-E reactor and no net H2 is produced.
A block diagram of the process is shown in FIG. 1. Referring to FIG. 1, air fed to separation unit 1 produces two streamsxe2x80x94oxygen having a purity of about 95% and a stream comprised primarily of nitrogen and argon. Alternatively, compressed air can be used without separation. The preheated oxygen or air, water (and/or steam) and a feedstock, e.g., petroleum coke, coal or heavy oil are fed to a partial oxidation reactor 2 which produces a stream composed of (primarily) hydrogen, carbon monoxide, carbon dioxide, water vapor, methane, ash and sulphur compounds. The ash is removed in separator 3 and the gases are cooled either by a direct water quench or by a waste heat boiler 4. The gases are further cooled in exchangers 5 and condensed water is removed in separator 6. Carbon dioxide and sulphur compounds are removed in acid gas removal unit 7, and hydrogen is removed in hydrogen removal unit 8. The gas remaining after hydrogen removal is primarily carbon monoxide which is fed along with steam to the K-E reactor 9. Products of the K-E reaction are separated in the product recovery unit 10. Tail gases from the product recovery unit contain a large percentage of carbon dioxide which can be removed in optional CO2 removal unit 11. The remaining tail gases can then be used as fuel for a gas turbine in a combined cycle electrical power plant 12, or for other plant fuel requirements. Carbon dioxide removed in units 7 and 11 can be sold as product or sequestered depending upon circumstances.
Note: Calculations were performed using proprietary in-house computer programs developed by Rentech (assignee of this invention) personnel to determine quantities of hydrogen and liquid hydrocarbons obtainable using the process of FIG. 1.